Datasheet

LTC2875
12
Rev A
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Supply Voltage Ranges
The LTC2875 can operate from 3.3V or 5V supplies. An inter-
nal comparator monitors the supply voltage and switches
internal reference voltages and output drive strengths at
approximately 4.1V. Because of the discontinuity in the
internal voltages at this switch point, operation with a sup-
ply voltage between 3.6V and 4.5V is not recommended.
±
60V Fault Protection
The LTC2875 addresses application needs for an overvolt-
age tolerant CAN transceiver that can operate from both
3.3V and 5V supplies and provide extended common mode
operation, high noise immunity and low electromagnetic
emission. Industrial installations may encounter com-
mon mode voltages between nodes far greater than the
–2V to 7V range specified by the ISO 11898-2 standard.
Competing CAN transceivers can be damaged by voltages
beyond their typical absolute maximum ratings of –3V to
16V. The limited overvoltage tolerance makes implemen
-
tation of effective external protection networks difficult
without interfering with proper data network performance.
Replacing standard CAN transceivers with LTC2875
devices can eliminate field failures due to overvoltage
faults without using costly external protection devices.
The ±60V fault protection of the LTC2875 is achieved by
using a high voltage BiCMOS integrated circuit technol
-
ogy. The naturally high breakdown voltage of this technol-
ogy provides protection in powered off and high imped-
ance conditions. The driver outputs use a progressive
foldback current limit to protect against overvoltage faults
while still allowing high current output drive.
The LTC2875 is protected from ±60V bus faults even
with the loss of GND or V
CC
(GND open faults are not
tested in production). In the case of V
CC
open, or shorted
to GND, the LTC2875 is off and the bus pins remain in
the high impedance state. Additional precautions must
be taken in the case of V
CC
present and GND open. The
LTC2875 chip protects itself from damage but may turn
on despite the open GND pin. When the RS or TXD input
is pulled low with the GND pin floating, a sneak path to
GND is established: through the ESD diode on the RS
or TXD pin; out through the RS or TXD pin; and into
the external driver that is pulling the pin low (Figure9).
APPLICATIONS INFORMATION
The current in this path can have a maximum current of
–100mA with a maximum voltage of approximately V
CC
– 2.5V during an overvoltage fault condition on the CANL
pin because the entire current that should flow out the
GND pin may flow out the input pin instead. If a GND open
fault with V
CC
present is anticipated, the system designer
should choose drivers for the RS and TXD inputs that are
protected against shorts to V
CC
– 2.5V.
The high voltage rating of the LTC2875 makes it simple to
extend the overvoltage protection to higher levels using
external protection components. Compared to lower volt-
age CAN transceivers, external protection devices with
higher breakdown voltages can be used so as not to inter-
fere with data transmission in the presence of large com-
mon mode voltages. Figure14 in the Typical Application
section shows a network capable of protecting against IEC
Level 4 surge, while still providing up to ±35V common
mode range on the signal lines.
Figure9. Sneak Path to GND with GND Pin Floating
±36V Extended Common Mode Range
The LTC2875 receiver features an extended common
mode range of –36V to 36V when operating from a 5V
supply, and –25V to 25V when operating from a 3.3V sup-
ply. The wide common mode increases the reliability of
operation in environments with high common mode volt-
ages created by electrical noise or local ground potential
differences due to ground loops. This extended common
mode range allows the LTC2875 to transmit and receive
under conditions that would cause data errors and pos
-
sible device damage in competing products.
2875 F09
RXD
TXD
TXD
GND
V
CC
RXD
V
CC
V
CC
LTC2875
RS
CANH
RSB
CANH
CANL
SPLIT
CANL
SPLIT
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